1. Field of the Invention
The present invention relates to a voltage-current conversion circuit and a switching regulator including the voltage-current conversion circuit.
2. Description of the Related Art
FIG. 6 is a circuit diagram for illustrating a related-art voltage-current conversion circuit 500.
The related-art voltage-current conversion circuit 500 includes a ground terminal 501, a power supply terminal 502, an input terminal 510, an error amplifier circuit 550, a resistor 552, an NMOS transistor 551, PMOS transistors 521 and 522, and an output terminal 528.
The error amplifier circuit 550 has a non-inverting input terminal connected to the input terminal 510, an inverting input terminal connected to one end of the resistor 552 and a source of the NMOS transistor 551, and an output connected to a gate of the NMOS transistor 551. Another end of the resistor 552 is connected to the ground terminal 501. The PMOS transistor 521 has a source connected to the power supply terminal 502, and a gate and a drain connected to a drain of the NMOS transistor 551. The PMOS transistor 522 has a source connected to the power supply terminal 502, a gate connected to the gate of the PMOS transistor 521, and a drain connected to the output terminal 528.
In the related-art voltage-current conversion circuit 500, the error amplifier circuit 550, the NMOS transistor 551, and the resistor 552 form a negative feedback circuit, which operates so that a voltage at the one end of the resistor 552 becomes equal to a voltage VIN at the input terminal 510.
As a result, a current I51 on a path of the resistor 552 is expressed by the following Expression (1), where a resistance value of the resistor 552 is represented by R.
                              I          ⁢                                          ⁢          51                =                  VIN          R                                    (        1        )            
In this way, according to the related-art voltage-current conversion circuit 500, the input voltage VIN is converted into the current I51, which is proportional to the input voltage VIN. Further, the PMOS transistors 521 and 522 form a current mirror circuit, to thereby output a current 152 proportional to the current I51 from the output terminal 528 (see, for example, Japanese Patent Application Laid-open No. 2012-200134).
The related-art voltage-current conversion circuit 500 as described above has a problem in that a period of time until the current I52 becomes a steady-state value after the voltage VIN is input, that is, a start-up period is long.
The cause of this problem is as follows. In general, an error amplifier circuit includes a phase compensation capacitor, and hence a phase compensation capacitor included inside the error amplifier circuit 550 is required to undergo a charging operation, which affects the start-up period. The phase compensation capacitor inside the error amplifier circuit is a basic circuit component that is publicly known, and therefore illustration thereof in the drawings is omitted.
In FIG. 7, a waveform of the output current I52 of the related-art voltage-current conversion circuit 500 is shown.
When a voltage VIN is applied at a time t0, the phase compensation capacitor begins to be charged, and, as shown in FIG. 7, the output current I52 increases at a constant slope. This slope is inversely proportional to a capacitance value of the above-mentioned phase compensation capacitor. Hence, although a degree of the slope differs depending on how large the capacitance value is, the gradual slope as shown in FIG. 7 is observed.
Therefore, a start-up period Ts5 from the time t0 at which the voltage VIN is input until a time is at which the current I52 becomes the steady-state value is long.